Using a stepwise method of multiple linear regression it was shown that high densities of macrobenthos are mostly explained by high quantities of plant debris. For meiofauna, together with plant debris, other effective variables are involved: silt content, organic carbon, chlorophyll pigments. On an average, a correlation between macro- and meiofaunal abundances could be found. However, this general pattern is modified by the biogenic structure created by the oysters and seagrass. When compared to the adjacent sandbanks, oysters clearly enhanced meiofaunal abundance (from 1130–4170 individuals 10 cm⁻²) but depressed macrofaunal densities (from 640-370 individuals 400 cm⁻²). The organic-rich oyster biodeposits probably favour meiofauna by an increase of the trophic resources but do not favour macroinfauna by inducing low oxygen concentrations. Moreover, it is likely that macrofauna is more sensitive to predation than meiofauna both in sandbanks and in oyster parks. For both meio- and macrofauna the highest incidences (7200 individuals 10 cm⁻² and 2470 individuals 400 cm⁻², respectively) are recorded in seagrass bed sediments. Zostera induces an enhancement of organic detritus and provides a refuge against predation. In terms of biomass, the macrofauna/meiofauna ratio is 25·1 in sandbanks, 1·5 in oyster parks and 4·2 in seagrass bed sediments. Macrofaunal biomass is more variable both spatially and temporally than meiofauna biomass. It is likely that the macrofauna is more sensitive to external factors such as predation, anoxia, exposure, than the meiofauna. Meiofauna abundance and biomass are more usually a function of food abundance and physical properties of the sediment.